Titanium materials such as pure titanium and titanium alloys have hitherto been used because of being excellent in specific strength, for example, in particular in the fields of aircrafts and vehicle parts requiring weight saving. The titanium material has a high in corrosion resistance and a high biocompatibility, and is used in various forms as constituent materials of biomedical implants.
However, there is a problem that these titanium materials have a low abrasion resistance, and tend to cause seizure; therefore, it has been difficult to use these titanium materials as sliding members. Accordingly, various surface treatment methods for improving the abrasion resistance of the titanium material have been developed. There is a method for forming a hardened nitrided layer on the surface of a titanium material as a hardening treatment method of the surface of a titanium material. As the method for forming a hardened nitrided layer on the titanium material, there have been known, for example, an ion nitriding treatment, a plasma nitriding treatment, and a thermal nitriding treatment.
In the ion nitriding treatment, for example, by using an ion implantation apparatus, in a low-pressure gas containing nitrogen and hydrogen, a glow discharge is generated by applying a direct current voltage of a few hundred volts across a titanium material and a furnace wall, and thus ionized N and NH form a hardened nitrided layer on the surface of the titanium material.
In the plasma nitriding treatment, for example, by using a high-frequency induction plasma generator, a plasma gas of nitrogen and hydrogen is introduced into a plasma torch section, and a titanium material is nitrided in the after-glow region to form a hardened nitrided layer on the surface of the titanium material.
However, the ion nitriding treatment and the plasma nitriding treatment require the use of a special apparatus such as an ion implantation apparatus or a high-frequency induction plasma generator; accordingly, in consideration of the simplicity of the treatment, the formation of a hardened nitrided layer by using the thermal nitriding treatment is effective.
The thermal nitriding treatment forms a hardened nitrided layer on the surface of a titanium material by holding the titanium material in a nitrogen gas at normal pressure and a high temperature for a few hours. For example, Non Patent Literature 1 discloses a technique of forming a hardened nitrided layer on the surface of a titanium material made of pure titanium. In Non Patent Literature 1, an annealed titanium material is hermetically sealed in a vacuum furnace, the vacuum furnace is evacuated to vacuum, the temperature is increased to and retained at a predetermined temperature (880° C.) while nitrogen gas is being allowed to flow at a flow rate of 1 L/min, and thus a nitriding is performed to form a hardened nitrided layer on the surface of the titanium material.
In addition to this, for example, Patent Literature 1 discloses a surface modification method of titanium or a titanium alloy for the purpose of forming a uniform and thick nitrided layer, “wherein titanium or a titanium alloy is heated in a hydrogen atmosphere, and allowed to absorb hydrogen in a content of 0.3 to 1.0 wt %, then heated in vacuum to be dehydrogenated to a hydrogen content of 0.01 wt % or less, thus allowed to have an activated state of the surface, and immediately subsequently subjected to a heating/cooling treatment to form a nitrided layer on the metal surface.”